Ok, easy guys, if I understand where you are going, a 50/50 mix or premixed coolant is your best bet. Tap water can vary in content. Some evidence shows tap water is ok as a mix with coolant. Coolant mix transfers heat better than water, every cooling system and engine will have it's own characteristics relative to temperature control. Forgetting about the protection that coolant provides as well as lubricating the water pump, yes straight water could get the job done in most circumstances.No reason to do this though?